With the ever-increasing utilization of electromagnetic waves in broadcasting, mobile communication, radar, cellular phones, wireless local area networks (LAN) and other systems, more electromagnetic waves are scattering in the ambient atmosphere. This situation frequently gives rise to the problems of electromagnetic wave disturbance and electronic equipment malfunction.
With the advance toward a higher density and higher integration of central processor units (CPU), micro processor units (MPU), large scale integrated circuits (LSI) other components used in electronic equipment such as personal computers and mobile phones as well as higher density mounting of electronic components on printed circuit boards, there arise problems that electromagnetic waves are radiated in the interior of equipment and reflected thereby so that the interior is full of electromagnetic waves. Electromagnetic interference can occur with the electromagnetic wave emitted by the equipment itself.
In the prior art, an artisan with specialized knowledge and experience of noise suppression must be engaged in taking a countermeasure against disturbances by electromagnetic interference. It is a time-consuming task to find an effective countermeasure. Another drawback is that a guarded component requires an extra space for mounting. To solve these problems, engineers are interested in electromagnetic absorbers which absorb electromagnetic waves for thereby reducing reflected and transmitted waves.
The drive toward higher density and higher integration of electronic equipment components such as CPU, MPU and LSI entails increased amounts of heat release. Ineffective cooling will allow thermal runaway, causing malfunction. One typical means for effectively radiating heat to the exterior is to place silicone grease or silicone rubber filled with heat conductive powder between electronic components (e.g., CPU, MPU and LSI) and heat sinks to reduce contact thermal resistance. This means, however, cannot avoid the problem of electromagnetic interference within the equipment interior.
Therefore, members having electromagnetic wave absorbing and heat transfer abilities are needed for high density and highly integrated components (e.g., CPU, MPU and LSI) within electronic equipment. Depending on the necessary situation, the state-of-the-art makes a choice among three types: (1) sheets of a base polymer with a magnetic powder dispersed therein having only an electromagnetic wave absorbing ability, (2) sheets of a base polymer with a heat conductive powder (like alumina) dispersed therein having only a heat transfer ability, and (3) sheets filled with both the powders having both electromagnetic wave absorbing and heat transfer abilities.
In these years, the signal processing speed of personal computers and other electronic equipment is drastically increasing. Many devices have an operating frequency of several hundred MHz to several GHz. Then electromagnetic noises generating in electronic equipment often have frequencies in the GHz band. To suppress such electromagnetic noises, the use of sheets having a spinel type cubic ferrite powder (typically manganese zinc base ferrite or nickel zinc base ferrite) uniformly dispersed in a base polymer may be useful. These ferrite sheets are effective mainly in the MHz band, but less in the GHz band. Then, sheets having uniformly dispersed in a base polymer a metal base soft magnetic powder which is more effective in the GHz band become the mainstream shield used at present.
Since soft magnetic metals are generally electrically conductive, sheets having such powder dispersed in a base polymer have a low breakdown voltage. Then, when a sheet is mounted within an electronic equipment, care must be taken so as to avoid short-circuits between individual parts with which the sheet can come in contact.
On use, the sheet having both electromagnetic wave absorbing and heat transfer abilities is often sandwiched between a device and a heat sink. The sheet cannot be used if electrical connection between a device and a heat sink is a problem. In this situation, an electrically insulating sheet having only a heat transfer ability is sandwiched between a device and a heat sink for dissipating the heat from the device. At the same time, a sheet having only an electromagnetic wave absorbing ability is placed at a nearby area where no electrical problem occurs, for suppressing electromagnetic noise. The use of two types of sheet is cumbersome.
Most of electromagnetic noise-generating sites within electronic equipment are high speed operating devices such as CPU, MPU and LSI, whereas pins or legs for connecting the device to the pattern of a printed circuit board and the printed circuit pattern itself can act as antennas to generate electromagnetic noises. In the latter case, it is preferred that an electromagnetic wave-absorbing material be directly mounted at the noise generating site. However, a material having uniformly dispersed in a base polymer a metal base soft magnetic powder which is fully effective to noise in the GHz band cannot be used because it causes short-circuits.
Basically, in the material having a metal base soft magnetic powder uniformly dispersed in a base polymer, the electroconductive soft magnetic metal particles are electrically insulated from each other since the base polymer is electrically insulating. To enhance the electromagnetic wave absorbing ability, the material must be heavily loaded with soft magnetic metal powder. As a consequence, metal particles are spaced only a close distance and can even be brought into contact, resulting in the composition having a lower breakdown voltage. JP-A 11-45804 describes an electromagnetic wave-absorbing material in which a soft magnetic metal powder is covered on the surface with an insulating coating of a silane coupling agent. JP-A 2001-308584 describes an electromagnetic wave-absorbing material in which a soft magnetic metal powder is covered on the surface with an insulating coating of a long-chain alkyl silane. The coatings of molecules having such organic radicals are difficult to provide an electromagnetic wave-absorbing composition with a satisfactory breakdown voltage.